A girl in Iowa regained her vision after losing it from severe complications of the flu.
Doctors said the blindness could have been permanent.
Jade's parents told CNN the girl had to be flown to the children's hospital at the University of Iowa on Christmas Eve.
She had a strain of the flu called Influenza B. Health experts said children are the most vulnerable to it.
Jade was unresponsive and relied on a ventilator in the pediatric intensive care unit.
"Influenza B is activating her own immune system to start attacking her own organs, specifically the brain and causing brain swelling," said Dr. Aditya Badheka.
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Doctors also feared Jade would not make it.
"It just hurts, and it rips your heart out because you just want her to wake up," said Jade's mom Amanda Phillips.
Two weeks later, Jade woke up, but doctors noticed that she couldn't see. They said the flu caused inflammation in her brain, and she may be blind forever.
"I didn't think I was going to see her again. At that point, I really didn't," Phillips said.
Jade went home from the hospital on Jan. 9. Her mom says both Jade and her sister received the flu shot in March 2019.
However, she didn't realize her daughters needed to be vaccinated again for the 2019-2020 flu season.
Since the flu virus changes year to year, the vaccine also changes.
Flu vaccines become available at the end of the summer, and the Centers for Disease Control and Prevention (CDC) recommends getting a flu shot by the end of October to protect against flu in the upcoming winter.
After weeks of treatment and recovery, Jade can see again and will start kindergarten soon.
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4-year-old girl who lost her sight to flu regains vision